1. Field of The Invention
The invention relates to construction materials and more particularly to shingles for roofing and siding applications on the exterior of a building.
2. Brief Description of the Related Art
Building panels useful as a roofing and siding material (shingles and clapboard) have been described as being thermoformed from polycarbonate resins; see for example U.S. Pat. No. 4,308,702. As is also described in this patent, the polycarbonate resin used in thermoforming can be compounded with fillers such as glass fibers and with other additives such as coloring pigments. Although polycarbonate resins can be formulated to provide molded articles with a wide range of properties advantageous to exterior building panels, resistance to ultra-violet light degradation is difficult to achieve, without sacrificing moldability. Useful quantities of ultra-violet absorbing compounds, when added to polycarbonate resins, particularly in the presence of coloring pigments, adversely affect the polymers melt rheology making melt flow difficult to control.
In spite of the difficulty in molding polycarbonate resins containing ultra-violet light resisting agents, polycarbonate resins do have other properties which make them valuable as wear surfaces. In fact, the U.S. Pat. Nos. 4,034,528 and 4,096,011 have suggested that they be used to surface clad vinyl resin exterior sidings. If they could be formulated to resist degradation from exposure to ultra-violet radiation without adversely affecting processability, a major advance in the art would follow. This did happen in regard to branched aromatic polycarbonate resins, wherein it was discovered that they were compatible with a specific class of ultra-light absorbing benzotriazole's; see U.S. Pat. No. 5,001,177. But branched or highly cross-linked polycarbonate resins, with or without resistance to ultra-violet radiation, are too brittle to be useful as an impact-resistant exterior building material, even as a surface cladding which is suggested in the '177 patent.